Complex fragmentation pathways of rhodanine and rhodanine-3-acetic acid upon resonant capture of low-energy electrons.

A detailed gas-phase study of dissociative electron attachment (DEA) to rhodanine (Rd) and its 3-acetic acid derivative (Rdaa) in the 0-14 eV energy range has been carried out with a magnetic mass spectrometer under conditions of medium energy resolution (0.4 eV) of the incident electron beam, and high sensitivity. The DEA spectra reveal the occurrence of numerous and complex dissociative decay channels of the molecular anions formed by resonances, involving multiple bond cleavage and structural re-arrangements. Along with a variety of anion fragments normally formed in the collision cell at incident electron energies < 1 eV, dissociation of a series of metastable anion species (occurring in the time scale of microseconds) is also detected. The observation of these slow processes is consistent with the complexity of many dissociative channels, and allows to propose schemes for the fragmentation pathways. The DEA spectra of both Rd and Rdaa also display small yields of molecular anions at zero energy. Their lifetimes with respect to electron detachment are experimentally evaluated to be about 30 microseconds and 200 microseconds, respectively. These (vibrationally excited) ground anion states formed by capture of thermal electrons mainly into the ring pi*(C=S) LUMO survive long enough to re-distribute their excess energy before undergoing different dissociation channels.